Engine piston and method of manufacturing the same

ABSTRACT

A method of manufacturing an engine piston may include performing upper-body formation for forming an upper body as an upper portion of a piston body by pressing a powder-type sintered material, performing lower-body formation for forming a lower body as a lower portion of the piston body by pressing a powder-type sintered material, performing bonding for forming the piston body by providing a brazing material between the upper body and the lower body and brazing the upper body and the lower body to each other while sintering a sintered material, performing machining for removing pores from the surface of the piston body by machining the surface, and performing heat treatment for forming a passive film by performing at least one of nitriding heat treatment or oxidation heat treatment on the surface of the piston body.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2018-0125496, filed on Oct. 19, 2018, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an engine piston and a method ofmanufacturing the same, and more particularly to an engine piston madeof an iron-based powder and a method of manufacturing the same.

Description of Related Art

In general, an engine piston of a vehicle is made of an aluminum alloymaterial.

Because a diesel engine is of a self-ignition type, the combustionefficiency is increased in proportion to the temperature of the upperside of the piston, that is, the temperature of the head portion.However, because the aluminum alloy material has very high thermalconductivity, the piston is easily cooled, thus making it difficult tokeep the temperature of the head portion high.

Therefore, a technology of changing the material of the piston from analuminum alloy material to a steel material to raise the temperature ofthe piston head portion has been developed. However, even when a pistonmade of a steel material is used, the maximum temperature of the pistonhead portion is slightly higher than that of a piston head portion madeof an aluminum alloy material, for example, 50 to 150° C. higher, andthe surface of the piston is oxidized and is subject to cracking.

Therefore, there is the demand for a new kind of engine piston capableof further raising the temperature of a piston head portion whilepreventing the occurrence of cracks, and a method of manufacturing thesame.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and may not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing anengine piston, which has excellent fatigue strength and oxidationresistance while further raising the temperature of a piston headportion by improving a thermal insulation property, and a method ofmanufacturing the same.

In accordance with an aspect of the present invention, the above andother objects may be accomplished by the provision of a method ofmanufacturing an engine piston, the method including performingupper-body formation for forming an upper body as an upper portion of apiston body by pressing a powder-type sintered material, performinglower-body formation for forming a lower body as a lower portion of thepiston body by pressing a powder-type sintered material, performingbonding for forming the piston body by providing a brazing materialbetween the upper body and the lower body and brazing the upper body andthe lower body to each other while sintering a sintered material,performing machining for removing pores from a surface of the pistonbody by machining the surface of the piston body, and performing heattreatment for forming a passive film by performing at least one ofnitriding heat treatment or oxidation heat treatment on the surface ofthe piston body.

In the performing the heat treatment, the oxidation heat treatment maybe performed after the nitriding heat treatment is performed on thesurface of the piston body.

The sintered material may include 0.45 to 0.6 wt % of C, 1.3 to 1.7 wt %of Cr, 0.15 to 0.35 wt % of Mo, 0.25 to 0.40 wt % of Mn, 0.1 to 0.2 wt %of S, and a remainder of Fe and inevitable impurities.

The sintered material may include 0.5 to 0.8 wt % of C, 0.8 to 0.9 wt %of Mo, 0.25 to 0.40 wt % of Mn, 0.1 to 0.2 wt % of S, and a remainder ofFe and inevitable impurities.

The brazing material used in the performing the bonding may include 35to 45 wt % of Cu, 5 to 30 wt % of one or more selected from the groupconsisting of Si, Mn, B, P, C and Fe, and a remainder of Ni.

In the performing the bonding, sintering and brazing may be performed at1100 to 1170° C. for 20 to 40 minutes, and the upper body and the lowerbody may be cooled to room temperature at a rate of 0.8 to 1.2° C./s.

In the performing the bonding, the upper body and the lower body may bespaced from each other by 0.05 to 0.15 mm, and the brazing material maybe provided between the upper body and the lower body.

In the performing the machining, the surface of the piston body may bemachined to a depth of 0.1 to 0.2 mm.

In the performing the heat treatment, the passive film may be formed byperforming oxidation heat treatment at 540 to 570° C. for 20 to 120minutes. The passive film may include a first oxide layer, which mayinclude Fe₃O₄ and is formed to have a thickness of 2 to 8 μm on thesurface of the piston body.

In the performing the heat treatment, a nitride layer may be formed byperforming nitriding heat treatment at 450 to 540° C. for 1 to 4 hours,and the passive film may be formed by performing oxidation heattreatment at 540 to 570° C. The passive film may include a nitridelayer, which may include iron nitride and is formed to have a thicknessof 4 to 20 μm on the surface of the piston body, and a second oxidelayer, which may include Fe₃O₄ and is formed to have a thickness of 1 to3 μm on the surface of the nitride layer.

In accordance with another aspect of the present invention, there isprovided an engine piston including a piston body formed by sintering anupper body and a lower body, formed through compression molding of apowder, brazing the upper body and the lower body to each othersimultaneously with the sintering, and machining a surface thereof toremove pores from the surface thereof, and a passive film formed on thesurface of the piston body, the passive film including at least one ofoxide or nitride.

The piston body may have a density of 6.9 to 7.3 g/cm³.

The piston body may have an internal porosity of 7.5 to 12%.

The passive film may include a first oxide layer, which may includeFe₃O₄ and is formed to have a thickness of 2 to 8 μm on the surface ofthe piston body.

The passive film may include a nitride layer, which may include ironnitride and is formed to have a thickness of 4 to 20 μm on the surfaceof the piston body.

The passive film may include a nitride layer, which may include ironnitride and is formed to have a thickness of 4 to 20 μm on the surfaceof the piston body, and a second oxide layer, which may include Fe₃O₄and is formed to have a thickness of 1 to 3 μm on the surface of thenitride layer.

The powder forming the piston body may include 0.45 to 0.6 wt % of C,1.3 to 1.7 wt % of Cr, 0.15 to 0.35 wt % of Mo, 0.25 to 0.40 wt % of Mn,0.1 to 0.2 wt % of S, and a remainder of Fe and inevitable impurities.

The powder forming the piston body may include 0.5 to 0.8 wt % of C, 0.8to 0.9 wt % of Mo, 0.25 to 0.40 wt % of Mn, 0.1 to 0.2 wt % of S, and aremainder of Fe and inevitable impurities.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an upper body and a lower body accordingto an exemplary embodiment of the present invention;

FIG. 2 is a cross-sectional view showing the state in which the upperbody and the lower body are brazed to each other;

FIG. 3 is a perspective view showing a piston body formed throughbonding of the upper body and the lower body;

FIG. 4 is a perspective view showing a final piston product obtained bymachining the piston body;

FIG. 5 is a schematic view showing the process of machining the surfaceof the piston body to remove pores from the surface of the piston body;and

FIG. 6 and FIG. 7 are schematic view showing a passive film formed onthe surface of the piston body.

It may be understood that the appended drawings are not necessarily toscale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particularly intendedapplication and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

The terminology used herein is for the purpose of describing variousexemplary embodiments only and is not intended to be limiting ofexemplary embodiments of the present invention. As used herein, thesingular forms “a,” “an,” and “the,” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “comprises,” “comprising,”“includes,” and/or “including,” when used herein, specify the presenceof stated features, regions, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, regions, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms used herein, which include technicalor scientific terms, have the same meanings as those generallyappreciated by those skilled in the art. The terms, such as ones definedin common dictionaries, should be interpreted as having the samemeanings as terms in the context of pertinent technology, and should notbe interpreted as having ideal or excessively formal meanings unlessclearly defined in the specification.

Hereinafter, an engine piston and a method of manufacturing the sameaccording to an exemplary embodiment of the present invention will bedescribed with reference to the accompanying drawings.

First, a method of manufacturing an engine piston according to anexemplary embodiment of the present invention will be described.

FIG. 1 is a perspective view of an upper body and a lower body accordingto an exemplary embodiment of the present invention, FIG. 2 is across-sectional view showing the state in which the upper body and thelower body are brazed to each other, FIG. 3 is a perspective viewshowing a piston body formed through bonding of the upper body and thelower body, FIG. 4 is a perspective view showing a final piston productobtained by machining the piston body, FIG. 5 is a schematic viewshowing the process of machining the surface of the piston body toremove pores from the surface of the piston body, and FIG. 6 and FIG. 7are schematic view showing a passive film formed on the surface of thepiston body.

As shown in FIGS. 1 to 7, an engine piston manufacturing methodaccording to an exemplary embodiment of the present invention broadlyincludes an upper-body-forming step, a lower-body-forming step, abonding step, a machining step and a heat treatment step.

As shown in FIGS. 1 to 4, the upper-body-forming step and thelower-body-forming step are steps of forming an upper body 110 and alower body 120, which may form a piston body 100. The upper body 110 andthe lower body 120 are formed through a compression molding method inwhich a powder-type sintered material is introduced into a mold and isthen compressed.

The upper body 110 forms the upper portion of the piston body 100, andthe lower body 120 forms the lower portion of the piston body 100. Theupper body 110 and the lower body 120 are coupled to each other to forma single piston body 100.

The upper body 110 may be a crown portion of the piston body 100, andmay have an insertion groove formed along the external circumferentialsurface thereof to receive a piston ring therein, and the lower body 120may be a skirt portion of the piston body 100.

The reason for forming the piston body 100 through assembly of the upperbody 110 and the lower body 120 is that it is difficult to form theoverall shape of the piston body 100 by filling a single mold with apowder-type material due to the structure of the piston body 100. Inother words, the piston body 100, which includes an undercut or a hollowregion, cannot be formed through a general powder compression moldingmethod, and is thus formed by assembling two separate parts.

The shapes of the upper body 110 and the lower body 120, formed in theupper-body-forming step and the lower-body-forming step, are notparticularly limited. However, as described below, the sintered materialforming the upper body 110 and the lower body 120 is a steel material toexhibit higher strength than the aluminum alloy used in the related art.Thus, the overall length of the piston body 100, which is formed throughassembly of the upper body 110 and the lower body 120, that is, thelength in the direction in which the piston body 100 ascends anddescends within a liner, may be reduced in comparison with the overalllength of a conventional piston made of an aluminum alloy material.

Accordingly, the length of the connecting rod may be increased, and thusthe angle formed between the connecting rod and the longitudinaldirection of the piston body 100 may be decreased. Furthermore, thelateral force applied to the piston body 100 may be decreased, and thusfriction between the piston body 100 and the internal wall of the linermay be minimized.

The sintered material forming the upper body 110 and the lower body 120may include 0.45 to 0.6 wt % of carbon (C), 1.3 to 1.7 wt % of chromium(Cr), 0.15 to 0.35 wt % of molybdenum (Mo), 0.25 to 0.40 wt % ofmanganese (Mn), 0.1 to 0.2 wt % of sulfur (S), and a remainder of iron(Fe) and inevitable impurities.

Alternatively, the sintered material may include 0.5 to 0.8 wt % ofcarbon (C), 0.8 to 0.9 wt % of molybdenum (Mo), 0.25 to 0.40 wt % ofmanganese (Mn), 0.1 to 0.2 wt % of sulfur (S), and a remainder of iron(Fe) and inevitable impurities.

The material of the upper body 110 may have the former composition, andthe material of the lower body 120 may have the latter composition.However, the present invention is not limited thereto. For example, anyone of the former composition and the latter composition may be used forboth the upper body 110 and the lower body 120.

Carbon (C) is a key element of the steel material, and is added withinthe above-mentioned range to increase the strength and stabilize thebainite structure.

Chromium (Cr) is added within the above-mentioned range to stabilize anitride layer and an oxide layer, which will be described later. When Cris added in an amount less than 1.3 wt %, the hardness of a passive filmlayer, which will be described later, is lowered. When Cr is added in anamount greater than 1.7 wt %, the formability in the compression moldingof the powder-type sintered material is lowered.

Molybdenum (Mo) is added within the above-mentioned range to improve thehigh-temperature strength through formation of carbide. When added withCr, Mo is alloyed together with Cr. When Mo is alloyed together with Crand when Mo is added in an amount less than 0.15 wt %, thehigh-temperature strength is lowered and the physical propertiesrequired of the engine piston cannot be attained, and when Mo is addedin an amount greater than 0.35 wt %, the strength is excessivelyincreased and powder molding becomes difficult. When Mo is used alonewithout Cr and when Mo is added in an amount less than 0.8 wt %, thehigh-temperature strength is lowered, and when Mo is added in an amountgreater than 0.9 wt %, powder formability is lowered.

Manganese (Mn) and sulfur (S) form spherical MnS, and are added in aform of MnS powder to improve processability. When Mn and S are added inan amount less than a lower limit, processability may be lowered, andwhen Mn and S are added in an amount greater than an upper limit,strength may be lowered or a weak portion may be formed due tosegregation of MnS.

In the bonding step, the above-described upper body 110 and lower body120 are sintered and brazed to each other simultaneously. The upper body110 and the lower body 120 are sintered by inserting a brazing material200 between the upper body 110 and the lower body 120 and heating andpressing the same. At the instant time, the brazing material 200partially penetrates the upper body 110 and the lower body 120, wherebythe upper body 110 and the lower body 120 are integrally bonded to eachother, resulting in formation of a single piston body 100.

The brazing material 200 may be formed in an annular shape. For example,the brazing material 200 may be formed in the shape of a washer, the topsurface of which is in contact with the bottom surface of the upper body110 and the bottom surface of which is in contact with the top surfaceof the lower body 120.

The brazing material 200 may include 35 to 45 wt % of Cu, 5 to 30 wt %of one or more selected from the group consisting of Si, Mn, B, P, C andFe, and a remainder of Ni. When Ni is present in an amount greater thanthe above-mentioned range, the melting point is increased, and theamount of Cu diffused between the upper body 110 and the lower body 120is decreased, whereby brazing is not properly achieved. When Cu ispresent in an amount greater than the above-mentioned range, thestrength-increasing effect obtained by Ni is lowered, and the bondingstrength of the brazed portion is thus lowered.

When the bonding step is performed, sintering and brazing are performedsimultaneously while the upper body 110 and the lower body 120 arepressed at 1100 to 1170° C. for 20 to 40 minutes. After the sinteringand brazing are completed, the upper body 110 and the lower body 120 arecooled to room temperature at a rate of 0.8 to 1.2° C./s, whereby thestructure of the piston body 100 is transformed into bainite.

When the bonding step is performed, the upper body 110 and the lowerbody 120 may be spaced from each other by a gap of 0.05 to 0.15 mm. Thegap may range from 0.06 to 0.12 mm. Due to the gap between the upperbody 110 and the lower body 120, the brazing material 200 easily flowsbetween the upper body 110 and the lower body 120 and is evenly diffusedbetween the bonding surface of the upper body 110 and the bondingsurface of the lower body 120.

When the gap is less than 0.05 mm, the space between the upper body 110and the lower body 120 is so narrow that the brazing material 200 is notproperly diffused. When the gap is greater than 0.15 mm, the brazing isnot realized, or the bonding strength does not satisfy 350 MPa.

To allow the upper body 110 and the lower body 120 to be spaced fromeach other, upper protrusions 111 are formed on the bonding surface ofthe upper body 110, and lower protrusions 121 are formed on the bondingsurface of the lower body 120. The upper body 110 and the lower body 120are spaced from each other by bringing the upper protrusions 111 and thelower protrusions 121 into contact with each other. The number of theupper protrusions 111 and the number of the lower protrusions 121 may bethe same, for example, three or more, to stably maintain the gap betweenthe upper body 110 and the lower body 120. Since the upper protrusions111 and the lower protrusions 121 are removed in the machining process,only the brazing material 200 is present between the upper body 110 andthe lower body 120, which are bonded to each other.

As shown in FIG. 5, in the machining process, the surface of the pistonbody 100 is machined to a predetermined machining depth 101 to removepores P from the surface.

Described in detail, when the surface of the piston body 100 ismachined, a pore P formed in the surface while having a size less thanthe machining depth 101 is completely removed. In the case of a pore Pformed in the surface while having a size greater than the machiningdepth 101, a portion of the surface of the piston body 100 around thepore P is introduced into the pore P, with the result that the entiresurface of the piston body 100 is flattened.

The machining depth 101 may range from 0.1 to 0.2 mm. When the machiningdepth 101 is less than 0.1 mm, it is difficult to remove pores P fromthe surface of the piston body 100. On the other hand, when themachining depth 101 is greater than 0.2 mm, the effect of removing poresfrom the surface of the piston body 100 is not improved Furthermore,machining costs are increased due to excessive machining, and the upperbody 110 and the lower body 120 need to be increased in size, thusincurring an increase in material costs.

Through the present surface-machining step, the surface of the pistonbody 100 is flattened by removing pores P from the surface of the pistonbody 100 or blocking pores P formed in the surface of the piston body100. However, pores P formed inside the piston body 100 are not removed,but serve as thermal insulation layers when the piston body 100 operatesas an engine piston.

After the machining process, the density of the piston body 100 mayrange from 6.9 to 7.3 g/cm³, and the internal porosity of the pistonbody 100 may range from 7.5 to 12%.

When the density is less than the above-mentioned range or when theinternal porosity is greater than the above-mentioned range, the thermalinsulation performance is improved, and the temperature of the pistonhead portion is therefore increased, improving the fuel efficiency ofthe diesel engine. However, the brittleness is increased and thelifespan is shortened. When the density is greater than theabove-mentioned range or when the internal porosity is less than theabove-mentioned range, the thermal insulation performance deteriorates,and it is therefore difficult to sufficiently raise the temperature ofthe piston head portion.

As shown in FIG. 6 and FIG. 7, in the heat treatment step, a passivefilm 130 is formed by performing at least one of nitriding heattreatment or oxidation heat treatment on the surface of the piston body100.

At the instant time, the detailed configuration of the passive film 130may vary depending on the heat treatment method.

As shown in FIG. 6, when only steam heat treatment is performed, thepassive film 130 may be configured as a first oxide layer 131, whichincludes Fe₃O₄ and is formed to have a thickness of 2 to 8 μm on thesurface of the piston body 100.

To form the first oxide layer 131 on the surface of the piston body 100,the oxidation heat treatment is performed at 540 to 570° C. for 20 to120 minutes in a steam atmosphere.

Since Fe₃O₄ is a porous material and has excellent thermal insulationproperty, it may help raise the temperature of the head portion of thepiston body 100. Furthermore, as described above, since the piston body100 according to an exemplary embodiment of the present inventionincludes chromium, Cr₂O₃ may be additionally formed in the first oxidelayer 131 through the steam heat treatment, improving the oxidationresistance of the surface of the piston body 100.

As shown in FIG. 7, when the nitriding heat treatment and the steam heattreatment are performed simultaneously, the passive film 130 may beformed to include a nitride layer 132, which includes iron nitride andis formed to have a thickness of 4 to 20 μm on the surface of the pistonbody 100, and a second oxide layer 133, which includes Fe₃O₄ and isformed to have a thickness of 1 to 3 μm on the surface of the nitridelayer 132.

At the instant time, the heat treatment is performed at 450 to 540° C.for 1 to 4 hours in an ammonia (NH₃) atmosphere to form the nitridelayer 132 including more than 50% of Fe₂₋₃N and a remainder of Fe₄N.Subsequently, the heat treatment is performed at 540 to 570° C. for 20to 120 minutes in a steam atmosphere to form the second oxide layer 133including Fe₃O₄.

When the nitride layer 132 is formed, an ion-nitriding method or agas-nitrocarburizing method may be used, and the porous nitride formedin the present manner may improve the thermal insulation property of thepiston body 100. Furthermore, after the nitriding heat treatment, theoxidation heat treatment may be performed in a steam atmosphere,improving the high-temperature fatigue strength of the piston body 100.

A dense layer having a dense structure is formed at a portion of thenitride layer 132 which is relatively close to the surface of the pistonbody 100, and a porous layer including pores is formed on the denselayer. The dense layer may be formed to have a thickness of 2 μm ormore, and the porous layer may be formed to have a thickness of 10 μm orless. The porous layer is configured to improve the thermal insulationproperty of the piston body 100, and the dense layer is configured toprevent oxidation of the piston body 100 by preventing permeation ofoxygen.

Alternatively, the passive film 130 may be formed using only the nitridelayer 132 without the second oxide layer 133. In the instant case, theoxidation heat treatment is not performed after the nitriding heattreatment.

To transform the piston body 100 shown in FIG. 3, which has undergonethe bonding step, into the final piston product 100 a shown in FIG. 4,additional machining may be performed.

This additional machining is performed to form an insertion groove, intowhich a piston ring is inserted, a pin hole, into which a connecting rodis fastened, and an oil gallery, in which a lubricant is stored.

This additional machining may be performed after the bonding step andbefore the machining step.

The engine piston according to an exemplary embodiment of the presentinvention includes a piston body 100, which is formed by sintering anupper body 110 and a lower body 120, formed through powder compressionmolding, bonding the upper body 110 and the lower body 120 to each otherusing a brazing material 200 simultaneously with the sintering, andmachining the surface thereof to remove pores from the surface thereof,and a passive film 130, which is formed on the surface of the pistonbody 100 and includes at least one of oxide or nitride.

Such an engine piston is manufactured by the above-described enginepiston manufacturing method, and the description of the concreteconfiguration thereof is substituted by the description of the enginepiston manufacturing method.

The engine piston manufactured according to the above-describedmanufacturing method has a compression height (the distance from the topsurface of the piston to the piston pin) of 25 to 40 mm, which isshorter than the compression height of a conventional piston made of Alby about 25%.

As is apparent from the above description, an engine piston and a methodof manufacturing the same according to an exemplary embodiment of thepresent invention have the following effects.

First, since the thermal conductivity is low, heat generated during fuelcombustion accumulates in a piston head portion, raising the temperatureof the piston head portion and consequently improving the combustionefficiency of the diesel engine.

Second, since oxidation resistance is excellent, it is possible toprevent the occurrence of cracks at high temperatures.

Third, since sintering and brazing are performed simultaneously, themanufacturing process is simplified.

Although exemplary embodiments of the present invention have beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the presentinvention as included in the accompanying claims.

For convenience in explanation and accurate definition in the appendedclaims, the terms “upper”, “lower”, “inner”, “outer”, “up”, “down”,“upper”, “lower”, “upwards”, “downwards”, “front”, “rear”, “back”,“inside”, “outside”, “inwardly”, “outwardly”, “internal”, “external”,“inner”, “outer”, “forwards”, and “backwards” are used to describefeatures of the exemplary embodiments with reference to the positions ofsuch features as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described toexplain certain principles of the invention and their practicalapplication, to enable others skilled in the art to make and utilizevarious exemplary embodiments of the present invention, as well asvarious alternatives and modifications thereof. It is intended that thescope of the invention be defined by the Claims appended hereto andtheir equivalents.

1-11. (canceled)
 12. An engine piston comprising: a piston body formedby sintering an upper body and a lower body, formed through compressionmolding of a powder, brazing the upper body and the lower body to eachother simultaneously with the sintering, and machining a surface of thepiston body to remove pores from the surface of the piston body; and apassive film formed on the surface of the piston body, the passive filmincluding at least one of oxide or nitride.
 13. The engine pistonaccording to claim 12, wherein the piston body has a density of 6.9 to7.3 g/cm³.
 14. The engine piston according to claim 12, wherein thepiston body has an internal porosity of 7.5 to 12%.
 15. The enginepiston according to claim 12, wherein the passive film comprises a firstoxide layer comprising Fe₃O₄, the first oxide layer being formed to havea thickness of 2 to 8 μm on the surface of the piston body.
 16. Theengine piston according to claim 12, wherein the passive film comprisesa nitride layer comprising iron nitride, the nitride layer being formedto have a thickness of 4 to 20 μm on the surface of the piston body. 17.The engine piston according to claim 12, wherein the passive filmcomprises: a nitride layer including iron nitride, the nitride layerbeing formed to have a thickness of 4 to 20 μm on the surface of thepiston body; and a second oxide layer including Fe₃O₄, the second oxidelayer being formed to have a thickness of 1 to 3 μm on an upper surfaceof the nitride layer.
 18. The engine piston according to claim 12,wherein the powder forming the piston body comprises 0.45 to 0.6 wt % ofC, 1.3 to 1.7 wt % of Cr, 0.15 to 0.35 wt % of Mo, 0.25 to 0.40 wt % ofMn, 0.1 to 0.2 wt % of S, and a remainder of Fe and inevitableimpurities.
 19. The engine piston according to claim 12, wherein thepowder forming the piston body comprises 0.5 to 0.8 wt % of C, 0.8 to0.9 wt % of Mo, 0.25 to 0.40 wt % of Mn, 0.1 to 0.2 wt % of S, and aremainder of Fe and inevitable impurities.